Characterization of transducers with small-aperture hydrophone and Schlieren system

Characterization of ultrasonic transducers and arrays is of great significance. The intensity or power irradiated by a transducer or array needs to be determined in order to avoid harmful biological effects on the human body. The beam pattern of a transducer determines the resolution and contrast of an imaging system. Side lobes and grating lobes produce artifacts and reduce the contrast of the imaging system.; Characterization of high frequency (>30 MHz) transducers and arrays is challenging due to spatial averaging, difficulties in alignment, attenuation and nonlinear propagation. Several small-aperture and broadband hydrophones are used to characterize high frequency transducers and arrays: a membrane hydrophone with a geometric spot diameter of 37 mum and a -3 dB bandwidth larger than 150 MHz, a PVDF needle hydrophone with a geometric spot diameter of 40 mum and a needle hydrophone with a geometric spot diameter of 150 mum.; A simple nonlinear experiment was used to estimate the bandwidths of these three small-aperture hydrophones and to evaluate their waveform distortions in a nonlinear field. It is shown that the HP membrane hydrophone produces the smallest distortions in waveform. Measurements of the ultrasonic fields of several high frequency single element transducers (30--60 MHz) and a 30 MHz annular array show that differences in pulse length and beam width among the hydrophones are reasonably related to the hydrophone's spot size and bandwidth. The measured beam profiles and beam widths also agree well with simulation.; A simple wire-target method was also studied in the high frequency range (35--60 MHz), with a performance comparable to that of a small-aperture hydrophone. It is a useful alternative to small-aperture hydrophones in characterizing lateral radiation patterns of single-element focused transducers.; In addition to the hydrophone and wire-target method, a faster and more convenient Schlieren method is used to visualize and map the acoustic fields from various low frequency (2--30 MHz) transducers and arrays. Tomographic reconstruction was compared with hydrophone measurement and shows good agreement. The application of the Schlieren system to phase aberration studies was also demonstrated. The results and comparisons with the hydrophone method demonstrate the Schlieren system's feasibility to visualize and analyze phase aberration.; These three methods provide comprehensive studies on the characterization of transducers and arrays.